1. Field of the Invention
The present invention relates to an optical head including a plurality of optical systems in which light beams of different wavelengths are used, and an optical recording and/or reproducing apparatus using the optical head. The present invention also concerns a tracking error signal detecting method using a plurality of optical systems.
2. Description of Related Art
During data recording and reproduction by an optical head with respect to information recording media, tracking error signal is detected as will be briefly described below concerning CD-ROM, DVD-ROM and recordable DVD, by way of example, as information recording media.
CD-ROM is a read-only digital optical disc. A light having a wavelength xcex of 780 nm is used to read data from the CD-ROM. DVD-ROM is a read-only digital optical disc designed for a higher recording density and larger storage capacity than CD-ROM. A light of about 635 nm or 650 nm in wavelength xcex is used for data read-out from DVD-ROM.
In addition to the conventional DVDs, a recordable digital optical disc has been proposed which is designed to have a higher recording density and larger storage capacity than CD-ROM. For data write and read-out with respect to this recordable DVD, a light having a wavelength xcex of about 635 nm or 650 nm is used. It should be noted that the recordable DVD includes mainly a one with respect to which data can be written but cannot be erased and a one with respect to which data can be written and also erased. These types of DVD will be generally called xe2x80x9crecordable DVDxe2x80x9d herein below.
For detection of tracking error signal in data read-out from CD-ROM, a pushpull signal detected by the pushpull method is used. The pushpull method is advantageous in that the optical system may be simple. On the other hand, however, it is disadvantageous in that a large offset takes place due to a lens shift, disc radial skew, and so forth. For the optical system for CD-ROM, the offset component has to be canceled through a correction by a processing circuit provided for the purpose.
That is, for use of the pushpull method, a correction circuit is necessary to cancel the offset component. It should be noted that the correction circuit will be referred to as xe2x80x9cpushpull offset correction circuitxe2x80x9d in the following description. In the conventional optical head, the pushpull offset correction circuit is incorporated in PDIC in the optical head. PDIC is an integration of a photodiode (PD) provided to detect a return light from the disc and an integrated circuit (IC) in which the light detection signal from the photodiode is processed in a predetermined manner.
Note that the read-only optical disc such as CD-ROM has pits formed on the recording surface thereof The pushpull signal cannot be detected if the pit is too deep. However, since the pit depth is prescribed to be about xcex/5 for CD-ROM, the pushpull signal can surely be detected from CD-ROM.
On the other hand, the pit depth is prescribed to be about xcex/4 for DVD-ROM. In this case, no pushpull signal can be detected. For detection of tracking error signal from DVD-ROM, for example, the heterodyne method is to be adopted. The heterodyne method is also called DPD (differential phase detection) method and a differential phase can be effected with a generally similar optical system to that in the optical head for the conventional CD-ROM. For the heterodyne method, however, the light detection signal has to be processed in a complicated manner by a processing circuit. In the following description, the circuit to effect this processing will be referred to as a xe2x80x9cheterodyne detection circuitxe2x80x9d
It should be noted however that the heterodyne method cannot be used for optical discs having no intensity modulation in the track direction, namely, which has no data written thereon. Therefore, the heterodyne method can be employed for optical discs having data previously written thereon such as DVD-ROM, but not suitable for use with any recordable optical discs.
However, it has been earnestly desired that DVD-ROM and recordable DVD are interchangeable with each other so that a single disc drive can be commonly used with them. In this situation, an optical disc has been proposed which can detect a tracking error signal by the heterodyne method when reading a DVD-ROM and by the pushpull method when writing or recording a recordable DVD. With such an optical head in a disc drive, it is possible to detect a tracking error signal in both DVD-ROM and recordable DVD so that DVD-ROM and recordable DVD are interchangeable with each other and selectively playable in the same disc drive.
Generally, a recordable DVD has grooves formed on the recording surface thereof. However, no pushpull signal can be detected if the grooves are too deep. However, if the groove depth is prescribed to be about xcex/8, pushpull signal can be detected without any problem.
With an optical head in which, to detect a tracking error signal, the heterodyne method is used for DVD-ROM while the pushpull method is employed for recordable DVD, a problem will arise that the processing circuit to process the light detection signal is rather complicated.
As mentioned above, a heterodyne detection circuit is indispensable for the complicated processing of the light detection signal when the heterodyne method is used, and a pushpull offset correction circuit is required to cancel offset component when the pushpull method is used. Therefore, for adoption of the heterodyne method for DVD-ROM and the pushpull method for recordable DVD, both the heterodyne detection circuit and pushpull offset correction circuit are required.
It is rather difficult to build both these circuits together in PDIC. Even if it is possible, a PDIC incorporating both the circuits will have a very large physical size. Therefore, it is desirable that a tracking error signal can be detected from both DVD-ROM and recordable DVD without any complicated processing circuit.
In the foregoing, the tracking error signal detection has been described concerning a one-spot method in which a single light beam is irradiated onto a disc. However, there is also available a so-called three-beam method (also called xe2x80x9ctwin-spot methodxe2x80x9d) in which three light beams are irradiated onto a disc to detect a tracking error signal.
FIG. 1 explains the possible problems encountered in detection of tracking error signal by the three-beam method, showing three light spots defined on a track when writing a new data.
When writing data initially into an area where no data is written on a recordable DVD, if the track pitch is very small as compared with the spot diameter, one side spot 101 will be positioned on a track T1 where data is already written while the other side spot 102 will be positioned on a track T2 where no data is written, as shown in FIG. 1. In this situation, the signal from the one side spot 101 and that from the other light spot 102 will be asymmetrical with each other so that no tracking error signal will be detected. Therefore, if the track pitch is extremely small in comparison with the spot diameter as in recordable DVD, the three-beam method cannot be adopted for the detection of tracking error signal.
In recordable DVD, the track pitch is very small, namely, less than a half of that in CD-ROM. Therefore, when the three-beam method is used for detection of a tracking error signal from DVD-ROM and recordable DVD, it is extremely difficult to align the optical head with a high accuracy. That is, since the track pitch in DVD-ROM and recordable DVD is very small, it is necessary to set, with a sufficiently high accuracy, an angle defined between the orientation of the three spots and the track, and thus it is necessary to align the optical head with a considerably improved accuracy. From this point of view, it is not desirable to adopt the three-beam method for detection of tracking error signal from DVD-ROM and recordable DVD.
It is therefore an object of the present invention to provide an optical head adapted to detect a tracking error signal from each of a plurality of different types of recording medium designed for high recording density and large capacity without using any complicated processing circuit.
It is another object of the present invention to provide an optical recording and/or reproducing apparatus using the optical head.
It is further objects of the present invention to provide a tracking error signal detecting method using the optical head.
The above object can be attained by providing an optical head in which a light is irradiated towards an information recording medium and a return light from the information recording medium is detected, comprising, according to the present invention:
a first optical system which irradiates a light of a first wavelength towards the information recording medium and detects a return part of the light reflected by the information recording medium; and
a second optical system which irradiates a light of a second wavelength shorter than the first wavelength towards the information recording medium and detects a return part of the light reflected by the information recording medium;
when writing or reading information with respect to the information recording medium using the second optical system, a pushpull signal being detected by the second optical system, and an offset component of the pushpull signal being canceled by a signal detected by the first optical system.
In the optical head according to the present invention, the second optical system is used to detect a pushpull signal and an offset component of the pushpull signal is canceled by the signal detected by the first optical system. Therefore, a pushpull signal whose offset component is canceled can be obtained with no pushpull offset correction circuit provided in the second optical system to cancel the offset component of the pushpull signal.
Note that the optical head may comprise an objective lens to condense light onto the information recording medium. The objective lens should preferably be usable in common by both the first and second optical systems. Thereby, the optical systems as a whole can be designed to be compact.
Also, the first and second optical systems of the optical head may each comprise an objective lens to condense light onto the information recording medium. In this case, these objective lenses should preferably be installed on a common lens driving means adapted to drive the objective lenses together. Thereby, even with the respective objective lenses of the first and second optical systems, the first optical system can be used to positively detect a necessary signal for canceling an offset component of a pushpull signal detected by the second optical system.
The above object can also be attained by providing an optical recording and/or reproducing apparatus in which light beams of first and second wavelengths are irradiated towards an information recording medium and a return light from the information recording medium is detected, comprising, according to the present invention:
a first optical system which irradiates a light of the first wavelength towards a first information recording medium and detects a return part of the light reflected by the first information recording medium to write and/or read information with respect to the first information recording medium; and
a second optical system which irradiates a light of the second wavelength shorter than the first wavelength towards a second information recording medium having a higher recording density than the first information recording medium and detects a return part of the light reflected by the second information recording medium to write and/or read information with respect to the second information recording medium;
when writing or reading information with respect to the second information recording medium using the second optical system, a pushpull signal of the return part of the light reflected by the second information recording medium being detected by the second optical system, and an offset component of the pushpull signal being canceled based on a signal of detection, by the first optical system, of the return part of the light reflected by the second information recording medium.
The above object can also be attained by providing a tracking error signal detecting method, comprising, according to the present invention, the steps of:
irradiating a light of a predetermined wavelength towards an information recording medium and detecting a return part of the light reflected by the information recording medium, by a first optical system;
irradiating a light of a shorter wavelength than that of the light used in the first optical system towards the information recording medium and detecting a return part of the light reflected by the information recording medium, by a second optical system;
detecting a pushpull signal as a tracking error signal by the second optical system; and
canceling an offset component of the pushpull signal with a signal detected by the first optical system.
In the tracking error signal detecting method according to the present invention, a pushpull signal is detected by the second optical system and an offset component of the pushpull signal is canceled with a signal detected by the first optical system. Therefore, a pushpull signal whose offset component is canceled can be obtained with no pushpull offset correction circuit provided in the second optical system to cancel the offset component of the pushpull signal.
These objects and other objects, features and advantages of the present intention will become more apparent from the following detailed description of the preferred embodiments of the present invention when taken in conjunction with the accompanying drawings.